1. Field of the Invention
The present invention relates to methods and apparatuses for characterization of a pulsed light beam.
2. Description of Related Art
In many instances where a laser beam is needed, it is important to know something about the laser beam quality. The beam quality affects how the beam will propagate, as well as how tightly it will focus. Unfortunately, beam quality is a somewhat elusive concept. Numerous attempts have been made to define beam quality, stretching back almost to the invention of the laser. In practice, any one of these measures will have some flaw in certain situations, and many different measures are often used. Among these is the M.sup.2 parameter (space-beamwidth product), Strehl ratio, root-mean-squared (RMS) wavefront error and power-in-the-bucket.
The irradiance (or intensity) and phase distribution of a laser beam are sufficient for determining how the beam will propagate or bow tightly it can be focused. Most of the beam quality measurements rely on characterizing the beam from only the irradiance distribution, since obtaining this is a comparably straightforward process. However, if both the irradiance and phase distribution could be obtained simultaneously, then all the information would be available from a single measurement.
In general, phase is measured with an interferometer. Interferometers are sensitive instruments that have been extensively developed. Interferometers can be configured in a shearing or filtered Mach-Zehnder arrangement to generate the desired information about the irradiance and phase distribution of a light beam. Unfortunately, these systems rapidly become complex, and are slow, unwieldy, sensitive to alignment, as well as being expensive. Other configurations for measuring wavefronts, such as a spatially varying transmittance ramp filter to divide light described in U.S. Pat. No. 4,690,555 to Ellerbroek, also require two entire separate sensors to spatially resolve the slope across the full aperture for a two-dimensional image. Such a requirement similarly adds complexity and expense.
A Shack-Hartmann wavefront sensor is an alternative method for measuring both irradiance and phase. Since at least 1971, such sensors have been developed by the military for defense adaptive optics programs. This sensor is a simple device that is capable of measuring both irradiance and phase distributions in a single frame of data. The advent of micro-optics technology for making arrays of lenses has allowed these sensors to become much more sophisticated in recent years. In addition, advances in charge coupled device (CCD) cameras, computers and automated data acquisition equipment have brought the cost of the required components down considerably. With a Shack-Hartmann wavefront sensor, determination of the irradiance and phase of a beam is relatively straightforward. This allows not only the derivation of various beam quality parameters, but also the numerical propagation of the sampled beam to another location, where various parameters can then be measured.
The M.sup.2 parameter has become a commonly used parameter to generally describe near-Gaussian laser beams. It is especially useful in that it allows a prediction of the real beam spot size and average irradiance at any successive plane using simple analytic expressions. This allow system designers the ability to know critical beam parameters at arbitrary planes in the optical system. Unfortunately, measuring M.sup.2 is somewhat difficult. To date, obtaining M.sup.2 has generally required measurements of propagation distributions at multiple locations along the beam path. Although attempts have been made to obtain this parameter in a single measurement, these attempts still suffer from the need to make simultaneous measurements at more than one location. The present invention permits calculation of this parameter of a pulsed light source using only a single measurement at a single location.
The following references relate to development of the present invention: A. E. Siegman, "New developments in laser resonators", SPIE Vol. 1224, Optical Resonators (1990), pp. 2-14; H. Weber, "Some historical and technical aspects of beam quality", Opt. Quant.Elec. 24 (1992), S861-S864; M. W. Sasnett, and T. F. Johnston, Jr., "Beam characterization and measurement of propagation attributes", SPIE Vol. 1414, Laser Beam Diagnostics (1991), pp. 21-32; D. Malacara, ed., Optical Shop Testing, John Wiley & Sons, Inc., 1982; D. Kwo, G. Damas, W. Zmek, "A Hartmann-Shack wavefront sensor using a binary optics lenslet array", SPIE Vol. 1544, pp. 66-74 (1991); W. H. Southwell, "Wavefront estimation from wavefront slope measurements", JOSA 70 (8), pp.993-1006 (August, 1980); J. A. Ruff and A. E. Siegman, "Single-pulse laser beam quality measurements using a CCD camera system", Appl. Opt., Vol.31, No.24 (Aug. 20, 1992) pp.4907-4908; Gleb Vdovin, LightPipes: beam propagation toolbox, ver. 1.1, Electronic Instrumentation Laboratory, Technische Universiteit Delft, Netherlands, 1996; General Laser Analysis and Design (GLAD) code, v. 4.3, Applied Optics Research, Tucson, Ariz., 1994; A. E. Siegman, "Defining the Effective Radius of Curvature for a nonideal Optical Beam", IEEE J. Quant.Elec., Vol.27, No.5 (May 1991), pp. 1146-1148; D. R. Neal, T. J. O'Hern, J. R. Torczynski, M. E. Warren and R. Shul, "Wavefront sensors for optical diagnostics in fluid mechanics: application to heated flow, turbulence and droplet evaporation", SPIE Vol.2005, pp. 194-203 (1993); L. Schmutz, "Adaptive optics: a modern cure for Newton's tremors", Photonics Spectra (April 1993); and D. R. Neal, J. D. Mansell, J. K Gruetzner, R. Morgan and M. E. Warren, "Specialized wavefront sensors for adaptive optics", SPIE Vol. 2534, pp. 338-348 (1995).
The present invention is of a wavefront sensor that is capable of obtaining detailed irradiance and phase values from a single measurement. This sensor is based on a microlens array that is built using micro optics technology to provide fine sampling and good resolution. With the sensor, M.sup.2 can be determined. Because the full beam irradiance and phase distribution can be predicted anywhere along the beam. Using this sensor, a laser can be completely characterized and aligned. The user can immediately tell if the beam is single or multi-mode and can predict the spot size, full irradiance, and phase distribution at any plane in the optical system. The sensor is straightforward to use, simple, robust, and low cost.
Thus, as noted above, the measurement of the phase and intensity of light beams is important for understanding their characteristics, propagation, for comparing the performance of different systems, and for predicting their characteristics at successive planes. There are many different methods for measuring the intensity distribution of a light beam, while measurement of the phase distribution is much more difficult. These difficulties increase when a pulsed light beam is to be characterized. Some existing instruments can make measurements of a series of light beam pulses assuming that the pulse-to-pulse variations are minimal. However, these very pulse-to-pulse variations are often of interest. Thus, to make these measurements, the phase and intensity of the laser must be measured in a single pulse.
The Shack-Hartmann wavefront sensor has been applied to laser phase measurement for several years. It has also recently been applied to measurement of laser beam intensity profile and beam quality in a single measurement, as disclosed in the '620 application. The Shack-Hartmann wavefront sensor has a number of advantages for phase measurement, including the ability to obtain all of the needed information for measurement of intensity and phase in a single measurement. However, the use of Shack-Hartmann wavefront sensors has not been applied to the intensity and phase measurement of pulsed light beams.
While some beam profilers have been developed for measuring pulsed laser beam intensity, these beam profilers do not measure the phase of the light. Since only the intensity of the light is measured, they are measuring a physically different quantity, and therefore are not sufficient for the measurement noted above.